Fingerprints have long been used for authentication purposes. While originally done for purposes of criminal investigation, in the electronic age, fingerprint detection has become a prevalent form of identification for, among others, security purposes. In such cases, the fingerprint pattern must be discerned and verified. In order to discern the particular fingerprint, fingerprint sensors are used to, in effect, generate a digital picture of the ridges and valleys that form the loops and whorls on a finger surface. This is done by having an array of cells, in which each corresponds to a single pixel of the fingerprint image. There are different types of sensors for doing so and their resolution is on the order of about 350 to 512 dpi, although higher pixel resolutions can be found. In order to accomplish the foregoing, capacitive sensors are commonly used.
One type of capacitive sensor uses one electrode for each pixel. The electrode measures the capacity relative to a neighbor electrode with the capacitance being different if a pixel is on a groove or on a ridge. Another type of capacitive sensor is similar to the previous one, except that capacitance is measured between the pixel and ground. Yet another type, more indicative of typical commercial systems, involves some combination of these two types. With still other types of capacitive sensors, the capacitance is measured using AC voltage on an inter-electrode and/or electrode to ground basis.
In some cases, the forgoing types of sensors can be used in a scanner-like configuration to obtain an image of the fingerprint using electrical current instead of light.
By way of background, FIG. 1 illustrates a simplified example of a portion 100 of a simple capacitive sensor. The sensor is made up of one or more semiconductor chips containing an array of individual cells 102a, 102b that are each smaller than the width of one ridge or valley on a finger, in the simplified example of FIG. 1, ˜50 μm. As illustrated in FIG. 1 and noted above, each cell 102a, 102b corresponds to a pixel and includes two conductor plates 104a, 104b, covered with an insulating layer 106 which acts as a capacitor dielectric.
The sensor is connected to an integrator that includes an inverting operational amplifier 108. The inverting amplifier 108 alters one current based on fluctuations in another current. Specifically, the inverting amplifier alters a supply voltage. The alteration is based on the relative voltage of two inputs, called the inverting terminal 110 and the non-inverting terminal 112. In this case, the non-inverting terminal is connected to ground, and the inverting terminal is connected to a reference voltage supply 114 and a feedback loop 116. The feedback loop 116, which is also connected to the amplifier output 118, includes the two conductor plates 104a, 104b. The two conductor plates 104a, 104b form a capacitor. The surface of the finger acts as a third capacitor plate and is separated from the two conductor palates by at least the insulating layer 106.
Since varying the distance between the capacitor plates changes the total capacitance of the capacitor, a greater capacitance will indicate a ridge 120 and a lesser capacitance will indicate a valley 122 (because of the air located in the valley) and thus, a ridge 120 will result in a different output signal from the cell than will result from the presence of a valley 122.
FIG. 2 illustrates, in overly simplified form, a top view of a capacitive sensor array 200 incorporating cells 202 such as, for example, those of FIG. 1 or some other cell design.
Each cell is typically addressed in a known manner, whether in parallel, in series, or some combination of the two (i.e. a scan), to obtain the image and, in some cases, to perform more complex operations like sub sampling. The output is provided to a analog signal processing circuitry (to allow for adjustment of gain and offset) and then to an A/D converter to convert the analog values to digital values. The digital values can then be processed as needed by, for example, an appropriately programmed microprocessor.
It should be appreciated that the above is somewhat overly simplified but conveys the general approach, the design, development and use of capacitive sensors, per se, being known and thus need not be elaborated on in greater detail for an understanding of the concepts described herein.
Ideally, for the best resolution, the sensor will be able to have direct contact with the finger to be read. However, in order to prevent damaging the sensor array from pressure, repeated usage or foreign substances that might be present in the vicinity or on a finger, a cover plate is placed over the sensor. This cover plate is typically glass and of sufficient thickness to resist the pressures of at least normal use and sufficient durability to allow for cleaning when necessary. However, the cover plate also increases the distance between the sensor plates and the finger, thereby reducing the sensitivity—and thus, accuracy—of the sensor. This is because, as should be evident from the above, the further the distance between the cover plate surface and the sensor, relative to the distance between the top of a ridge and the bottom of a valley, the more difficult it becomes to discriminate between ridges and valleys.
Thus, there is a need in the art for an improved capacitive fingerprint sensor that allows for use of a sufficiently strong and durable cover plate while also allowing for detection of the ridges and valleys needed for accurate fingerprint capture.